This invention relates to a method and apparatus for examining a pattern capable of producing an output which represents whether an examined pattern is a predetermined pattern or not, or whether it is located at a predetermined position or not. The invention is especially suitable for examining a minute pattern such as a pattern of a mask utilized to prepare a semiconductor integrated circuit device or a pattern formed by such mask and utilized to fabricate a semiconductor integrated circuit device.
Typical methods for examining such a minute pattern as that of a semiconductor integrated circuit device are a chip-chip comparing method disclosed, for example, in IEEE Transaction on Electron Device, Vol. ED-22, No. 7, July 1975, PP487-495; a mask-mask comparing method, a design pattern data comparing method and a spatial filtering method disclosed, for example, in Solid State Technology, May, 1978, PP51-59; and a pattern analyzing method disclosed, for example, in Microelectronics and Reliability, Vol. 15, PP 585-593, Pergamon Press, 1976, printed in Great Britain.
According to the chip-chip comparing method, adjacent chips on the same mask are enlarged and compared with two optical systems to find out a coincidence, whereas according to the mask-mask comparing method a working mask is compared with a master mask. In each of these two methods, two optical systems are used for finding out coincidence of the images so that the limit of the defect detection is 2 to 3 microns and the setting of the masks is difficult. According to the design pattern data comparing method, an original design pattern data utilized for preparing a reticle is compared with a mask to be examined, whereas according to the spatial filtering method a crisscross shaped spatial filter is installed on the focal plane of a magnifying lens to intercept the diffraction light caused by a normal pattern so as to transmit only the diffraction light caused by a defect, thereby detecting the same. According to the pattern analyzing method, an magnified image of a mask is converted into a video signal by an image pickup tube or the like, and the video signal is checked by a method analogous to the pattern recognition so as to judge as a defect an irregular portion different from a pattern of LSI, etc.
Since each of the design pattern comparing method, the spatial filtering method, and the pattern analyzing method utilizes a single optical system, it is possible to pickup an image at a high resolution. However, the design pattern comparing method and the spatial filtering method are not inherently suitable for the detection of minute defects. More particularly, according to the prior art design pattern comparing method, it is extremely difficult to set the mask so that there are such difficulties as a mask setting error, distortion of the image and increase in the data. For this reason, these methods are not suitable to examine minute patterns. Although the spatial filtering method is easy to set a mask, the S/N ratio is small. Accordingly, only the pattern analyzing method is suitable for examining minute or micro patterns of LSI or the like.
In the examination of the LSI pattern, it is essential not only to examine the minute pattern but also to detect loss of pattern which occurs when a pattern is transferred from a reticle to a master mask. Since the minimum pattern width of the mask for an LSI is determined by a pattern rule so that it is necessary to detect a defect less than a width determined by this pattern rule. The pattern analyzing method may be used for this purpose, whereas the design pattern data comparing method may be used for detecting the loss of pattern and large defects. Accordingly, it has long been desired to provide a method of examining a pattern capable of rapidly and accurately examining a minute pattern and defective loss of pattern.